pass through headers on sdk responses

[bkendall]

Description

When fetching the JS SDKs through the Hosting emulator, we should pass through the headers we receive (including content-type).

Fixed #3041

Scenarios Tested

Before:

bkend-macbookpro2:tmp.SnkkGg9A » curl -I localhost:5000/__/firebase/8.2.2/firebase-analytics.js
HTTP/1.1 200 OK
Date: Thu, 14 Jan 2021 22:33:09 GMT
Connection: keep-alive
Keep-Alive: timeout=5

After

bkend-macbookpro2:tmp.SnkkGg9A » curl -I localhost:5000/__/firebase/8.2.2/firebase-analytics.js
HTTP/1.1 200 OK
accept-ranges: bytes
vary: Accept-Encoding
content-type: text/javascript; charset=UTF-8
cross-origin-resource-policy: cross-origin
content-length: 35654
date: Fri, 08 Jan 2021 01:29:41 GMT
expires: Sat, 08 Jan 2022 01:29:41 GMT
last-modified: Thu, 07 Jan 2021 21:51:27 GMT
x-content-type-options: nosniff
server: sffe
x-xss-protection: 0
cache-control: public, max-age=31536000
age: 594483
alt-svc: h3-29=":443"; ma=2592000,h3-T051=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"
Connection: keep-alive
Keep-Alive: timeout=5

[coveralls]

Coverage decreased (-0.02%) to 65.524% when pulling a2429ae on bk-3041 into 933ef1b on master.