(bug 43177) API: Fix regression in case handling for sha1 params

While the change in Ic980fd71 makes sense since MediaWiki stores the
hashes with lowercase letters, historically aisha1 and fasha1 accepted
both upper and lowercase because wfBaseConvert accepts both.

To avoid a backwards compatibility break, let's adjust the code to
explicitly lowercase the input for those parameters. And for good
measure, let's also accept both cases for the corresponding sha1base36
parameters.

Change-Id: I704935193398c722c22a302dc9d23f2c0e2e3e39