Fix whitespace in TemplateParserTester

Mixed tabs and spaces got switched to just spaces.
TemplateParser return string when keys missing

TemplateParser now returns the string it is parsing even when there are
missing variables, making variables optional.
TemplateParser keys not optional and missing keys not replaced
Document an existing TemplateParser feature with new test

Apparently template-ception is a supported feature of TemplateParser.
Add a new test to document and cover regression of this feature.