(bug 48864) json decode value for addQualifier in SetQualifier api module

Before this patch, SetQualifier accepted a non-json encoded string
for $params['value'] and failed to handle any json-encoded values
like wikibase-item and time data types.

This makes the module work for adding qualifiers with all data types,
and makes it consistent with the CreateClaim module.

Also updated the test to test for additional data value type.

Change-Id: I6c5f02aec5d87b413f896412d1b8f26dcbb3e895
Merge "(bug 48864) json decode value for addQualifier in SetQualifier api module"