Merge branch 'release/3.1.2'
Upgrade to beat the rule of solr ...

Example of "add" with optional update attribute to set a field to null (i.e. delete a field):

<add>
  <doc>
    <field name="employeeId">05991</field>
    <field name="skills" update="set" null="true" />
  </doc>
</add>
Merge pull request #188 from andrelemos/develop

Upgrade to beat the rule of solr ...
testcases for #188
Replacing relative classes namespaces in PHPDocs by absolute ones to improve IDE support.
Return null explicitely in methods and PHPDoc.
Add a 'use' statement to avoid FQCN in the code.
Improve PHPDocs (adding missing @throws, @return etc).
Fixing bad PHPDocs for methods.
Removed unused lines of codes.
Fix more PHPDocs.
Throw a RuntimeException in case the facet identifier is unknown.
Add type hints for dynamically-created instances.
Allow optional MoreLikeThis parameters
Merge remote-tracking branch 'gnutix/phpstorm-inspection-fixes' into develop (PR #192)

Conflicts:
	library/Solarium/QueryType/Select/Result/MoreLikeThis/Result.php
Merge pull request #196 from jchampion/optional-mlt-params

Optional moreLikeThis parameters
Added testcase for PR #196
Fixing type hints on Document that should hints on DocumentInterface.
Add strong type hinting on the DocumentInterface for 'getDocument(s)'-like methods.
Implement DocumentInterface in Solarium\QueryType\Analysis\Query\Document.