Move override application logic to configuration class

While the normal expected use case for an override is to apply it to a request, there are uses cases where we might want to apply an override to a non-request context. For example, we have a script that runs outside of the main rails app to generate some static configs we copy/paste into some static files.

It would look something like this:

```
_, static_csp_policy = SecureHeaders::CSP.make_header(SecureHeaders::Configuration.get.apply_overrides(:static_file_policy).csp, nil)
...
%q(<meta http-equiv="Content-Security-Policy" content="#{static_csp+policy"})
```

Author: ptoomey3

lib/secure_headers.rb

@@ -165,13 +165,9 @@ def header_hash_for(request)
     #
     # name - the name of the previously configured override.
     def use_secure_headers_override(request, name)
-      if override = Configuration.overrides(name)
-        config = config_for(request)
-        config.instance_eval(&override)
-        override_secure_headers_request_config(request, config)
-      else
-        raise ArgumentError.new("no override by the name of #{name} has been configured")
-      end
+      config = config_for(request)
+      config.apply_override(name)
+      override_secure_headers_request_config(request, config)
     end
 
     # Public: gets or creates a nonce for CSP.

lib/secure_headers/configuration.rb

@@ -167,6 +167,18 @@ def dup
       copy
     end
 
+    # Public: Apply a named override to the current config
+    #
+    # Returns self
+    def apply_override(name)
+      if override = self.class.overrides(name)
+        instance_eval(&override)
+      else
+        raise ArgumentError.new("no override by the name of #{name} has been configured")
+      end
+      self
+    end
+
     def generate_headers(user_agent)
       headers = {}
       HEADERABLE_ATTRIBUTES.each do |attr|