Although the spec[1] is defined in such a way that a trailing semi-colon
is valid it also doesn't allow a semi-colon by itself to indicate an
empty policy. Therefore it's easier (and valid) just to omit it rather
than to detect whether the policy is empty or not.
[1]: https://www.w3.org/TR/CSP2/#policy-syntax
Setting up the request environment was accidentally creating a CSP
as a consequence of accessing the option - only set the instance
variable if a block is passed.
This reverts commit 86f7c26907, reversing
changes made to 5ece2e4a44.
If a policy is set then we should generate it even if it's empty.
However what is happening is that we're accidentally generating an
empty policy when the initializer is commented out by default.
`Rails.application.config.content_security_policy` is configured with no
policies by default. In this case, Content-Security-Policy header should
not be generated instead of generating the header with no directives.
Firefox also warns "Content Security Policy: Couldn't process unknown
directive ''".
