Fix fetch_sdss_spectrum for HTTPS SDSS downloads - #285

Fixes #284.

fetch_sdss_spectrum currently fails against the SDSS endpoint with HTTP Error 308: Permanent Redirect because the downloader still uses http. Switching the URL to https also exposed a second issue in url_content_length(), which assumes a specific header casing for Content-Length.

This patch:

• updates the SDSS spectrum URL to use https
• makes url_content_length() read the header in a case-insensitive-safe way
• updates SDSSfits so https://... sources are treated as remote FITS URLs
• adds regression tests for all three behaviors

Testing:

• added regression tests in astroML/datasets/tests/test_datasets.py