Fixed failing integration tests.

This commit fixes a set of failing integration tests: 

* Verifying an app push was timing out periodically. Increased the wait time before giving up on a successful push. 
* File retrieval tests were failing because of a bad path in the test, due to a change in how the Java buildpack stages apps.
* Retrieving an empty file returns a null String from the API because of `Content-length: 0` in the HTTP header, not a zero-length string as the test expected. 
* Parsing of dates in the meta-data was broken, probably due to a change in the format of dates in the REST responses. Fixed this bug in the API and added more tests for date parsing.

All integration tests pass locally with these changes.