⚡️ Simplify header-fld-name parser

This speeds up my (earlier) benchmarks by ~15-20% over 0.4.3, and by
22-40% over earlier versions.  (FYI: I have not re-run the benchmarks
against v0.4.16 or v0.5.0-dev.)

NOTE: Previously, Net::IMAP recreated the raw original source string.
Now, it returns the decoded astring value.  Although this is technically
incompatible, it should almost never make a difference: all standard
header field names are valid atoms.

Where it _does_ make a difference, it should simplify client code, by
parsing (normalizing) the result and allowing clients to ignore server
inconsistency.

This will also allow clients to simply rely on FetchData#header for
extracting the result.  Without this commit, FetchData#header won't work
when servers return quoted or literal strings and the user expected
an atom.

(https://www.iana.org/assignments/message-headers/message-headers.xhtml)

Author: nevans

lib/net/imap/response_parser.rb

@@ -1317,31 +1317,19 @@ def header_list
       #   header-fld-name = astring
       #
       # NOTE: Previously, Net::IMAP recreated the raw original source string.
-      # Now, it grabs the raw encoded value using @str and @pos.  A future
-      # version may simply return the decoded astring value.  Although that is
-      # technically incompatible, it should almost never make a difference: all
-      # standard header field names are valid atoms:
+      # Now, it returns the decoded astring value.  Although this is technically
+      # incompatible, it should almost never make a difference: all standard
+      # header field names are valid atoms:
       #
       # https://www.iana.org/assignments/message-headers/message-headers.xhtml
       #
-      # Although RFC3501 allows any astring, RFC5322-valid header names are one
-      # or more of the printable US-ASCII characters, except SP and colon.  So
-      # empty string isn't valid, and literals aren't needed and should not be
-      # used.  This is explicitly unchanged by [I18N-HDRS] (RFC6532).
-      #
-      # RFC5233:
+      # See also RFC5233:
       #     optional-field  =   field-name ":" unstructured CRLF
       #     field-name      =   1*ftext
       #     ftext           =   %d33-57 /          ; Printable US-ASCII
       #                         %d59-126           ;  characters not including
       #                                            ;  ":".
-      def header_fld_name
-        assert_no_lookahead
-        start = @pos
-        astring
-        end_pos = @token ? @pos - 1 : @pos
-        @str[start...end_pos]
-      end
+      alias header_fld_name astring
 
       # mailbox-data    =  "FLAGS" SP flag-list / "LIST" SP mailbox-list /
       #                    "LSUB" SP mailbox-list / "SEARCH" *(SP nz-number) /

test/net/imap/fixtures/response_parser/fetch_responses.yml

@@ -70,7 +70,7 @@
       data: !ruby/struct:Net::IMAP::FetchData
         seqno: 10
         attr:
-          "BODY[HEADER.FIELDS (\"Content-Type\")]": "Content-Type: multipart/alternative;\r\n
+          "BODY[HEADER.FIELDS (Content-Type)]": "Content-Type: multipart/alternative;\r\n
             boundary=\"--==_mimepart_66cfb08b4f249_34306b61811e5\"\r\n\r\n"
       raw_data: *test_fetch_msg_att_HEADER_FIELDS_quoted