Lowercase scheme in regexp test

This works in requests 1.1 and 2.0 but it fails in the 1.2 series. It's
not something we should be too concerned about enforcing at the
requests-mock layer so just fix up the test.

Closes-Bug: #1361497
Change-Id: I8f30262ab3177d9e3d3644c083aed6031eeba78f