Escape special characters in the XML generated in the delete() method

[ramayer]

Type pysolr .delete() method was not escaping special characters like <, >, and & in the XML it was generating.

In particular attempts to delete documents by IDs like this:

    self.solr.delete(id='cats<dogs')

would give error messages, and attempts to delete documents like this:

    self.solr.delete(q='id:*</query><query> id:999 AND id:9999')

could delete more than you would have expected. This pull request (with test cases) escape the IDs and Lucene Query Expressions in the XML generation step.

[ramayer]

Note that this has the same functionality as [#358].

This version took the approach of using minimal changes building XML the same way.

The other version took the approach of changing Delete to use ElementTree to build the XML instead of python string manipulation.

I have no preference which approach is approved -- they both generate the same XML.

[ramayer]

Based on comments in [#357], the different implementation of this fix under pull request [#358] is a better approach.