spec/unit_tests/serializer: remove cruft (inspired by rubocop)

[ndushay]

Please review especially for:

• no useful tests removed (some pointless ones have been removed)
• no existing test functionality has been changed (refactored some variable names or avoided long lines or changed otherwise for readability)

This spec cleanup:

• shortens names of specs to be just the method names
• removes comments containing the rdoc or implementation of methods
• removes specs that are useless (e.g. that setting an attribute value means you can retrieve the attribute value; that instantiating an object returns an instance of the object)
• splits out multiple tests in a single spec into separate tests
• avoids long lines
• refactors variable names for clarity
• removes unnecessary @ variables
• tried to follow principles to make rubocop happier
  etc.

Connected to #6

[coveralls]

Coverage increased (+0.002%) to 99.319% when pulling 613ab12 on clean-serializer-specs into beb7dfa on master.

[jmartin-sul]

one pretty minor quibble, but otherwise, looks good to me.

[jmartin-sul]

seems like this should be filename instead of pathname here and in the prior test name?

[ndushay]

good catch - thanks. fixing now.

[coveralls]

Coverage increased (+0.002%) to 99.313% when pulling 1a311a6 on clean-serializer-specs into aca484d on master.