Use ChannelCredentials instead of ServerCredentials

[samtstern]

Description

Fixes #1404

Scenarios Tested

I used the scenarios in #1404

Before

HTTP/1.1 500 Internal Server Error
Connection: keep-alive
Content-Length: 562
Content-Type: text/html; charset=utf-8
Date: Mon, 17 Jun 2019 20:28:00 GMT
ETag: W/"232-MredztaPFklKVY1xHRYuRLVPYZc"
X-Powered-By: Express

Error: Getting metadata from plugin failed with error: this.channel.credentials._getCallCredentials is not a function
    at Http2CallStream.call.on (/tmp/tmp.ZkMEM0XGPF/functions/node_modules/@grpc/grpc-js/build/src/client.js:101:45)
    at emitOne (events.js:121:20)
    at Http2CallStream.emit (events.js:211:7)
    at process.nextTick (/tmp/tmp.ZkMEM0XGPF/functions/node_modules/@grpc/grpc-js/build/src/call-stream.js:71:22)
    at _combinedTickCallback (internal/process/next_tick.js:132:7)
    at process._tickCallback (internal/process/next_tick.js:181

After

HTTP/1.1 200 OK
connection: keep-alive
content-length: 15
content-type: application/json; charset=utf-8
date: Mon, 17 Jun 2019 20:31:52 GMT
etag: W/"f-dSg84JWK6uB1DoIr+zeTnpfFbQo"
x-powered-by: Express

{
    "data": "🔥"
}

Sample Commands

N/A

[coveralls]

Coverage remained the same at 61.369% when pulling c5ff075 on ss-fix-1404 into 82bae0a on master.

[ryanpbrewster]

nice find

[samtstern]

The fix for this issue is available in version 7.0.1