[BUGFIX] Fix exception header in InternalRequestEngine
When using the virtual browser in functional tests, the
InternalRequestEngine communicates any exceptions
which occurred during the request by adding special
headers to the HTTP response.
Previously the "X-Flow-ExceptionCode" header contained
exception codes like "#1222268609: ". This patch fixes
the rendering to just the code, like "1222268609".
Releases: 2.0, master