Raise an error if mobile: true is specified along with size

Currently we automatically resize to mobile dimensions if `mobile:
true` is specified.  This can be misleading if the user also passes
`height` and/or `width`, because those would be disregarded.

Raising an error makes the behaviour more obvious.

Author: duncan-bayne

lib/ferrum/page.rb

@@ -186,6 +186,10 @@ def set_viewport(width:, height:, scale_factor: 0, mobile: false)
     end
 
     def resize(width: nil, height: nil, fullscreen: false, mobile: false)
+      if mobile && (height != 0 || width != 0)
+        raise "specifying `mobile: true` causes automatic resizing to mobile dimensions"
+      end
+
       if fullscreen
         width, height = document_size
         self.window_bounds = { window_state: "fullscreen" }

spec/page_spec.rb

@@ -284,6 +284,22 @@ def is_mobile?
           page.reload
         end.to change { body_size }.to(width: 980, height: 2120)
       end
+
+      context "when a height is passed" do
+        it "raises an error" do
+          expect do
+            page.resize(width: 0, height: 100, mobile: true)
+          end.to raise_error("specifying `mobile: true` causes automatic resizing to mobile dimensions")
+        end
+      end
+
+      context "when a width is passed" do
+        it "raises an error" do
+          expect do
+            page.resize(width: 100, height: 0, mobile: true)
+          end.to raise_error("specifying `mobile: true` causes automatic resizing to mobile dimensions")
+        end
+      end
     end
   end
 end