Remove adding prefix to custom parameters.
Fix a malformed custom_parameter bug.
Fix custom parameter.
Revert "Fix a malformed custom_parameter bug."

This reverts commit 75bdc1eebfd444bf00102b486a881f5776bbc407.
Fix comments.
Merge pull request #1291 from edx/anton/fix-lti-custom-parameters

LTI: Fix bugs related to custom parameters.