Resolve ambiguity between descriptorFile and fileName when parsing descriptors

In 1.9.0 a new method getDescrptorFile was introduced. This actually can cause various problems and has some open design questions that should be answered in this ticket. (I'll attach a branch with a test demonstrating part of the problem.)

DescriptorParser has the new method (since 1.9.0):

public Iterable<Descriptor> parseDescriptors(byte[] rawDescriptorBytes,
    File descriptorFile, String fileName);

In addition to the problem demonstrated in the (to be attached) test this raises some questions:

1. What should be done, if descriptorFile.getName() != fileName?
2. What should be done, if descriptorFile != null && (null == fileName || fileName.isEmpty())?
3. What should be done, if descriptorFile == null && null != fileName? 4a) What should be done, if descriptorFile == null && null != fileName && fileName.isEmpty()? 4b) What should be done, if descriptorFile == null && null == fileName?

The implementation of the solution should also have tests for each of the possible cases.