Fixes content-type checking for image uploading in API v1 and v2

wsgi.Request.get_content_type() need a list or set for
allowed_content_types as parameter, but API controller passes request's
content type as a string into it directly. This caused 'in' testing
failure.

Fixes bug: 1241387

Change-Id: I820ccce92e4ec1c94b5144309d6ebe2b57baaf55
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Fixes content-type checking for image uploading in API v1 and v2"