Overly strict sanity check on post() method of DatasetCollectionHandler

Given that it is now possible to post with `?host=datafile.root_domain`, the test in line 2108:

``` python
if self.request.uri != '/datasets':
msg = "Method not Allowed: invalid datasets post request"
log.info(msg)
raise HTTPError(405, reason=msg) # Method not allowed
```

is too strict since the suffix of the URI can legitimately be something other than `/datasets`...

Start in DatasetCollectionHandler at the post() sanity check around line 2108 and inspect how request.uri is formed when using ?host=datafile.root_domain. Verify the valid POST is not rejected solely because its URI differs from /datasets, while the resulting method validation still behaves correctly.

Written by the indexing model from the issue text.