🔥 Feature: Add ReqHeaderParser.

[efectn]

Benchmarks:

Old

Benchmark_Ctx_Body_With_Compression-4      	  849862	      1233 ns/op	     145 B/op	       7 allocs/op
Benchmark_Ctx_BodyParser_JSON-4            	 4025719	       313.4 ns/op	      32 B/op	       2 allocs/op
Benchmark_Ctx_BodyParser_XML-4             	  314671	      3698 ns/op	    1160 B/op	      24 allocs/op
Benchmark_Ctx_BodyParser_Form-4            	  464664	      3494 ns/op	     360 B/op	      13 allocs/op
Benchmark_Ctx_BodyParser_MultipartForm-4   	  715200	      2160 ns/op	     328 B/op	      12 allocs/op
Benchmark_Ctx_QueryParser-4                	  244102	      4782 ns/op	     856 B/op	      38 allocs/op
Benchmark_Ctx_QueryParser_Comma-4          	  226207	      5551 ns/op	     928 B/op	      44 allocs/op

New

Benchmark_Ctx_Body_With_Compression-4             899160              1345 ns/op             145 B/op          7 allocs/op
Benchmark_Ctx_BodyParser_JSON-4                  4736698               260.0 ns/op            32 B/op          2 allocs/op
Benchmark_Ctx_BodyParser_XML-4                    363993              3336 ns/op            1160 B/op         24 allocs/op
Benchmark_Ctx_BodyParser_Form-4                   679537              1649 ns/op             360 B/op         13 allocs/op
Benchmark_Ctx_BodyParser_MultipartForm-4          729796              1525 ns/op             328 B/op         12 allocs/op
Benchmark_Ctx_QueryParser-4                       220290              5403 ns/op             856 B/op         38 allocs/op
Benchmark_Ctx_QueryParser_Comma-4                 204379              6221 ns/op             928 B/op         44 allocs/op
Benchmark_Ctx_ReqHeaderParser-4                   199575              6579 ns/op             928 B/op         44 allocs/op