Fix rendering outside a web request in non TEST env (#60)

[billgonemad]

Fixes #60

Problem

Since 364bd4e, RenderEnvironment only bound a web request in the TEST environment. In production/development, rendering off-request (jobs, async tasks, BootStrap) NPE'd as soon as a template used a taglib.

Fix

• No request bound: binds a GrailsWebRequest over Grails' servlet-free PageRenderer stubs (the same ones groovyPageRenderer uses). It uses the real context path and runs request-scoped cleanup.
• Request already bound: reuses the caller's request, so session, flash, params and request attributes keep working. Only out, the locale resolver and WrappedResponseHolder are swapped, and they are restored afterwards.
• The caller's request state is always restored, including on exceptions and nested renders.

Behaviour change

Off-request, session, without grails.serverURLthrow UnsupportedOperationExceptionin all environments. They already NPE'd in production on 7.0.x, but in TEST they used to work, so downstream test suites may notice. Setgrails.serverURL`.

Testing

New integration specs force grails.env=production, since TEST hid the bug. 14 of the 18 new tests fail against 7.0.x. ./gradlew check build docs :example:integrationTest: 90 tests, 0 failures.