parseURL() breaks custom Proxy

[canning-duck]

really this is #147, but we're using a custom Proxy and the behavior of parseURL() breaks our code as we pass URL's String() to the function that actually searches for the proxy (Java's URI()/ ProxySelector.Select(), we're running on Android using gomobile)

To restate the issue: the connection URL is "wss://example.com/im/connect?foo=bar".

• parseURL() returns &url.URL{Scheme:"wss", Opaque:"/im/connect", Host:"example.com", RawQuery:"foo=bar"} that resolves to "wss:/im/connect?foo=bar" when calling String(). The problem is that the host is missing.
• net/url.Parse() would return &url.URL{Scheme:"wss", Host:"example.com", Path:"/im/connect", RawQuery:"foo=bar"} that resolves to "wss://example.com/im/connect?foo=bar" as expected (go 1.9).

[garyburd]

The parse function works around a bug in Go 1.4 and earlier. See https://go.googlesource.com/go/+/874a605af0764a8f340c3de65406963f514e21bc for the detals.

It's OK to drop support for Go 1.4. I will accept a PR that replaces parseURL() with url.Parse().