This library is a fork of https://github.com/jarcoal/httpmock, which now has a somewhat different API from the original library. All of the clever stuff was worked out in the original version, this fork just adds some extra fussiness in how it registers and responds to requests.
The reason for creating this fork was that while the original httpmock library provided a very neat mechanism for inserting mocked responses into the default Go net/http client, it was intentionally designed to be very tolerant about the requests it matched which lead to us seeing some bugs in production.
To give a specific example let's say our code requires requesting data from some external resource (http://api.example.com/resource/1), and for correct operation it must include in that request a specific authorization header containing a value passed in from somewhere else within the program. For this scenario the previous library would let us create a responder for the resource http://api.example.com/resource/1, but wouldn't allow us to force a test failure if that resource was requested without the authorization header being present.
For usage examples, please see the documentation at https://godoc.org/github.com/thingful/httpmock.