added header customizability to graphql.Request

[jsonkao]

This PR gives developers the ability to add their own headers to a graphql.Request. The graphql.Request.Header mirrors the Header of a http.Request. It contains the request header fields sent by the client.

If a server received a request with header lines,

Host: example.com
accept-encoding: gzip, deflate
Accept-Language: en-us
fOO: Bar
foo: two

then

Header = map[string][]string{
  "Accept-Encoding": {"gzip, deflate"},
  "Accept-Language": {"en-us"},
  "Foo": {"Bar", "two"},
}

[jsonkao]

+1

[matryer]

@jkao1 Is there a reason why you didn't just use http.Header instead of making your own?