Change single char string literals to chars (#2304)

Author: Skylion007

httplib.h

@@ -5398,7 +5398,7 @@ bool read_content(Stream &strm, T &x, size_t payload_max_length, int &status,
 inline ssize_t write_request_line(Stream &strm, const std::string &method,
                                   const std::string &path) {
   std::string s = method;
-  s += " ";
+  s += ' ';
   s += path;
   s += " HTTP/1.1\r\n";
   return strm.write(s.data(), s.size());
@@ -5407,7 +5407,7 @@ inline ssize_t write_request_line(Stream &strm, const std::string &method,
 inline ssize_t write_response_line(Stream &strm, int status) {
   std::string s = "HTTP/1.1 ";
   s += std::to_string(status);
-  s += " ";
+  s += ' ';
   s += httplib::status_message(status);
   s += "\r\n";
   return strm.write(s.data(), s.size());
@@ -5682,9 +5682,9 @@ inline std::string params_to_query_str(const Params &params) {
   std::string query;
 
   for (auto it = params.begin(); it != params.end(); ++it) {
-    if (it != params.begin()) { query += "&"; }
+    if (it != params.begin()) { query += '&'; }
     query += encode_query_component(it->first);
-    query += "=";
+    query += '=';
     query += encode_query_component(it->second);
   }
   return query;
@@ -6443,9 +6443,9 @@ inline std::string make_content_range_header_field(
 
   std::string field = "bytes ";
   field += std::to_string(st);
-  field += "-";
+  field += '-';
   field += std::to_string(ed);
-  field += "/";
+  field += '/';
   field += std::to_string(content_length);
   return field;
 }