Overwrite host and webroot when forcessl is enabled

This patch enables the use of forcessl together with a multiple domains
reverse SSL proxy (owncloud/core#1099) which have different hostname
and webroot for http and https access. The code assumes that the ssl
proxy (https) hostname and webroot is configured via overwritehost and
overwritewebroot.
request.php: add type check to the not empty check of a string

The not equal comparison (<>) of a variable with an empty string
could lead to false positive results as the compare do not check
the type and thereby could not make sure that the checked variable
is a string. The usage of the not identical comparison operator
(!==) make sure that the variable is a string and not empty.
Merge pull request #3893 from owncloud/backport_1872

Backport #1872