Stub Endpoints only on specific HTTP-Methods

[ybonjour]

When a double is created for an endpoint it currently stubs the endpoint for all HTTP methods.
It should be possible to create a double only for specific HTTP methods.

Something along the lines of :

spy.endpoint('/foo').should_return_error_code(500) # Stubs all HTTP methods
spy.endpoint('/foo', GET).should_return_error_code(500) # Stubs only GET
spy.endpoint('/foo', [GET, POST]).should_return_error_code(500) # Stubs GET and POST