Is this a bug report?

Yes

Have you read the Contributing Guidelines?

Yes

Environment

1. react-native -v: 0.42.0
2. node -v: v6.11.2
3. npm -v: 4.6.1
4. yarn --version: 0.27.5

• Target Platform: iOS
• Development Operating System: macOS
• Build tools: XCode

Steps to Reproduce

1. Try to run a react native app and download a bundle from a server which returns a Content-Type header that includes both the MIME Type and an optional character encoding as specified here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type
2. The React Native code for iOS will fail on this line of code as it strictly checks the Content-Type header as if the optional character encoding doesn't exist: https://github.com/facebook/react-native/blame/master/React/Base/RCTJavaScriptLoader.mm#L277

Expected Behavior

The code should follow the specification of Content-Type and respect only MIMEType checking instead of strictly checking the Content-Type header for an exact match on MIMEType.

Per Apple documentation, NSURLResponse contains the MIMEType field that should be checked for application/javascript instead of checking the raw Content-Type header: https://developer.apple.com/documentation/foundation/nsurlresponse

Actual Behavior

Though this was encountered while using Expo, the error is nonetheless coming from the RCTJavaScriptLoader

Expected JavaScript, but got content type 'application/javascript; chartset=UTF-8'. Check that your internet connection is working.

Reproducible Demo

https://github.com/sysgears/apollo-universal-starter-kit

Follow starter instructions and checkout this commit: sysgears/apollo-universal-starter-kit@bbcd52a

The problem is worked around after this commit in the project.