fix:disallow that a method don't match regEnLetter

[bestgopher]

if I difine a custom method aaa, it can be registerd by Match, but can not by Handle.
The Handle will check the method by regEnLetter regexp rule.
I think the two ways should be consistent.

package main

import "github.com/gin-gonic/gin"

func main() {
	d := gin.Default()

	// use Match to register aaa will be ok.
	d.Match([]string{"aaa"}, "/aaa", func(c *gin.Context) {
		c.String(200, "aaa")
	})

	// use Handle to register aaa will panic.
	// panic: http method aaa is not valid
	d.Handle("aaa", "/aaa", func(c *gin.Context) {
		c.String(200, "aaa")
	})

	d.Run(":8080")
}

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c2ba8f1) 99.20% compared to head (d592e50) 99.20%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3727   +/-   ##
=======================================
  Coverage   99.20%   99.20%           
=======================================
  Files          42       42           
  Lines        3157     3157           
=======================================
  Hits         3132     3132           
  Misses         17       17           
  Partials        8        8           

Flag	Coverage Δ
	99.20% <100.00%> (ø)
go-1.18	99.11% <100.00%> (ø)
go-1.19	99.20% <100.00%> (ø)
go-1.20	99.20% <100.00%> (ø)
macos-latest	99.20% <100.00%> (ø)
ubuntu-latest	99.20% <100.00%> (ø)

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Files Changed	Coverage Δ
routergroup.go	100.00% <100.00%> (ø)

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