Expose event message headers, introduce a new way to read the body from an io.Reader

[m19c]

This pull request introduces the changes described in #1944.

• Expose the constants sha1SignatureHeader, sha256SignatureHeader, eventTypeHeader and deliveryIDHeader
• Introduce a new public function called ValidatePayloadFromBody to read the body directly from an io.Reader rather than a http.Request.

The changes described enables the possibility to use go-github even with http libraries like fiber or fasthttp.

Example A:

app.Get(func(ctx *fiber.Ctx) error {
	signature := ctx.Get(github.SHA256SignatureHeader)
	if signature == "" {
		signature = ctx.Get(github.SHA1SignatureHeader)
	}
	
	payload := ctx.Body()
	err := github.ValidateSignature(signature, payload, mySecretToken)
	// ...

	event, err := github.ParseWebHook(ctx.Get(github.EventTypeHeader), payload)
	// ...
})

Example B:

app.Get(func(ctx *fiber.Ctx) error {
	signature := ctx.Get(github.SHA256SignatureHeader)
	if signature == "" {
		signature = ctx.Get(github.SHA1SignatureHeader)
	}
	
	payload, err := github.ValidatePayloadFromBody(ctx.Get("Content-Type"), ctx.Context().RequestBodyStream(), signature, mySecretToken)
	// ...

	event, err := github.ParseWebHook(ctx.Get(github.EventTypeHeader), payload)
	// ...
})

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[m19c]

@googlebot I signed it!

[gmlewis]

Thank you, @m19c !
LGTM.

Awaiting second LGTM before merging.

[codecov]

Codecov Report

Merging #1955 (329d1c9) into master (a7a3d8c) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1955      +/-   ##
==========================================
+ Coverage   97.84%   97.87%   +0.02%     
==========================================
  Files         105      105              
  Lines        6809     6809              
==========================================
+ Hits         6662     6664       +2     
+ Misses         80       79       -1     
+ Partials       67       66       -1     

Impacted Files	Coverage Δ
github/messages.go	100.00% <100.00%> (+2.94%)	⬆️

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[gmlewis]

@m19c - do you want to resolve the merge conflicts, or would you like me to do that?

[gmlewis]

Thank you, @m19c ! Could you please run gofmt one more time and push the changes to make the tests pass?

[m19c]

Done. Intent should be fixed now.

[gmlewis]

@m19c - bonus points if you can increase the code coverage by writing a test for your new method (while we are waiting for a second LGTM). 😄

[m19c]

Done 👍🏽

[m19c]

I've also added another test to capture invalid application/x-www-form-urlencoded requests.
The messages.go file now has 100% test coverage.

[Parker77]

LGTM.