BUG: Fix error path in HTTPEndPoint

[dopplershift]

We were assuming that Content-Type was always available in the headers, which isn't the case when NCSS fails.

This changes:

Traceback (most recent call last):
  File "250hPa_Hemispheric_Plot.py", line 46, in <module>
    data = ncss.get_data(gfsdata)
  File "/Users/rmay/repos/siphon/siphon/ncss.py", line 114, in get_data
    resp = self.get_query(query)
  File "/Users/rmay/repos/siphon/siphon/http_util.py", line 379, in get_query
    return self.get(url, query)
  File "/Users/rmay/repos/siphon/siphon/http_util.py", line 457, in get
    if resp.headers['Content-Type'].startswith('text/html'):
  File "/Users/rmay/miniconda3/envs/py36/lib/python3.6/site-packages/requests/structures.py", line 54, in __getitem__
    return self._store[key.lower()][1]
KeyError: 'content-type'

to

Traceback (most recent call last):
  File "250hPa_Hemispheric_Plot.py", line 46, in <module>
    data = ncss.get_data(gfsdata)
  File "/Users/rmay/repos/siphon/siphon/ncss.py", line 114, in get_data
    resp = self.get_query(query)
  File "/Users/rmay/repos/siphon/siphon/http_util.py", line 379, in get_query
    return self.get(url, query)
  File "/Users/rmay/repos/siphon/siphon/http_util.py", line 463, in get
    text))
requests.exceptions.HTTPError: Error accessing http://thredds.ucar.edu/thredds/ncss/grib/NCEP/GFS/Global_0p5deg/GFS_Global_0p5deg_20180223_1200.grib2?var=u-component_of_wind_isobaric&var=v-component_of_wind_isobaric&var=Geopotential_height_isobaric&time=2018-02-23T22%3A28%3A49.823819&west=0&east=360&south=0&north=90&accept=netcdf4&addLatLon=True&vertCoord=25000: 400 Requested time 2018-02-23T22:28:49.823Z does not intersect actual time range 2018-02-26T06:00:00Z - 2018-02-26T06:00:00Z

[jrleeman]

Another corner case down!