http.Header create helper for convenience

[cyucelen]

Problem:

When asserting http headers, it is verbose to create expected http headers.

Such as:

expectedHeader := http.Header{}
expectedHeader.Add("X-If-You-Read-This", "send-a-hadouken-back")
requestRecorder.AssertHeaderEqual(t, expectedHeader)

Solution:

A wrapper for creating http.Header and adding the initial key-value pair can be written for conciseness.

Such as:

requestRecorder.AssertHeaderEqual(t, aduket.ExpectedHeader("X-If-You-Read-This", "send-a-hadouken-back"))