Fixed #279. Rewrite CSPPlugin.

What has changed?

+ tests are completly rewritten.
+ CSPPlugin is completely rewritten to support 1.0 syntax.
+ Compare to the previous CSSPlugin, CSP and XCSP presence check is always required.

The orginal rewrite expected more issues to raise, but I have given up that idea, Instead,
only relevant and basic syntax checks are done. Issues/summaries like how many directives
inherit from default-src are not available (although they were implemented I backed out those
codes during squash). Also, this plugin currently only checks the syntax of the CSP header; so
XCSP, CSP-RO and XCSP-RO are not verified. An experimental CSPPlugin (but separate) should
be developed. Until then this plugin can remain simple like this.