test: refactor _ContentTypeMap construction test
opc: add CaseInsensitiveDict for overrides
opc: add case insensitive test for defaults
opc: refactor _ContentTypesItem

such that its .xml_for() method instantiates it and uses that instance
to calculate the correct XML.
opc: factor leading period out of PackURI.ext
release: v0.3.1 hotfix issue #76

Failed to load certain presentations containing images with uppercase
filename extension.