Bug fixes and debug output

 - Fixed ordering of merge_docs
 - Fixed precedence merge behaviour
 - Merge type and type_name
Increased error message verbosity
Added PROTECTED_KEYS to only merge keys we want

Adds special consideration for keys special to our implementation
(Solr) such as the id / version which we cannot change.
Added test to be sure we merge docs correctly
Added specific test for ordering of merge_docs
Reverted error message / paginated indexing
Unused variable
Merge pull request #62 from ox-it/importer-precedence-fix

Importer precedence fix