Fix RESTDataSource fetching a path with a colon

[brandon-leapyear]

RESTDataSource.resolveUrl doesn't work correctly for paths with a colon, e.g.

this.get('/foo:')

this throws

TypeError: Only absolute URLs are supported

The source code strips out the leading slash, which leads to the call new URL('foo:', 'http://example.com'), where foo: looks like an absolute URL without a host.

https://github.com/apollographql/apollo-server/blob/cd19cfeff260c4b948fb56f0b3b210a43858cc8c/packages/apollo-datasource-rest/src/RESTDataSource.ts#L74-L88

[brandon-leapyear]

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---

Suggested solution: change resolveURL to something like

let prefix = '', baseURL = undefined
if (this.baseURL) {
  const base = new URL(this.baseURL)
  const basePath = base.pathname
  prefix = basePath.endsWith('/') ? basePath : basePath.concat('/')
  baseURL = base.origin
}

return new URL(prefix + path, baseURL)

[glasser]

Duplicate of #23. Will pull your suggestion over there.