Explicitly set the content-type on HTTPReceiver responses to the /slack/install route

[filmaj]

Summary

While most browsers will automatically infer the content-type header even if not set on HTTP responses, certain cloud hosting systems (e.g. AWS API Gateway) integrate with a variety of other services to provide content and have no means of inferring the content type. As a result, deploying bolt apps to AWS and integrating with API Gateway can be difficult.

This PR explicitly sets the Content-Type header to text/html for HTTPReceiver /slack/install route responses.

This PR fixes #1279.

[codecov]

Codecov Report

Merging #1280 (bd95235) into main (ba64593) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head bd95235 differs from pull request most recent head f1bf78b. Consider uploading reports for the commit f1bf78b to get more accurate results

@@            Coverage Diff             @@
##             main    #1280      +/-   ##
==========================================
+ Coverage   73.22%   73.24%   +0.01%     
==========================================
  Files          17       17              
  Lines        1438     1439       +1     
  Branches      431      431              
==========================================
+ Hits         1053     1054       +1     
  Misses        300      300              
  Partials       85       85              

Impacted Files	Coverage Δ
src/receivers/HTTPReceiver.ts	51.06% <100.00%> (+0.20%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ba64593...f1bf78b. Read the comment docs.