Re-instate granular tests for #24
Re-instate granular tests for #24

Re-instated each of:

- testIsMultipart()
- testGetHeader()
- testGetDecodedHeader()
- testGetHeaderAsArray()

each with the `filesProvider` data provider; these replace the newly added
`testParseFile()` in order to provide better granularity of assertions and
behavior.

mleko/zend-mail,MadCat34/zend-mail,Xerkus/zend-mail,rapidsoft-de/zend-mail