[AIRFLOW-626] HTML Content does not show up when sending email with attachment

[illop]

Dear Airflow Maintainers,

Please accept this PR that addresses the following issues:

• https://issues.apache.org/jira/browse/AIRFLOW-626

Per Apache guidelines you need to create a Jira issue.

Testing Done:

• Unittests are required, if you do not include new unit tests please
  specify why you think this is not required. We like to improve our
  coverage so a non existing test is even a better reason to include one.

Reminders for contributors (REQUIRED!):

• Your PR's title must reference an issue on
  Airflow's JIRA.
  For example, a PR called "[AIRFLOW-1] My Amazing PR" would close JIRA
  issue Improving the search functionality in the graph view #1. Please open a new issue if required!

• For all PRs with UI changes, you must provide screenshots. If the UI changes are not obvious, either annotate the images or provide before/after screenshots.

• Please squash your commits when possible and follow the How to write a good git commit message.
  Summarized as follows:

  1. Separate subject from body with a blank line
  2. Limit the subject line to 50 characters
  3. Do not end the subject line with a period
  4. Use the imperative mood in the subject line (add, not adding)
  5. Wrap the body at 72 characters
  6. Use the body to explain what and why vs. how

[codecov-io]

Current coverage is 65.96% (diff: 66.66%)

Merging #1885 into master will increase coverage by 0.05%

@@             master      #1885   diff @@
==========================================
  Files           129        129          
  Lines          9898       9899     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits           6524       6530     +6   
+ Misses         3374       3369     -5   
  Partials          0          0          

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