Output warning instead of raising an exepection on default type mismatch

See #435 (comment)

Closes #533.

Author: sferik

lib/thor/parser/option.rb

@@ -127,7 +127,8 @@ def validate_default_type!
         @default.class.name.downcase.to_sym
       end
 
-      raise ArgumentError, "An option's default must match its type." unless default_type == @type
+      # TODO: This should raise an ArgumentError in a future version of Thor
+      warn "Expected #{@type} default value for '#{switch_name}'; got #{@default.inspect} (#{default_type})" unless default_type == @type
     end
 
     def dasherized?

spec/parser/option_spec.rb

@@ -135,9 +135,9 @@ def option(name, options = {})
   end
 
   it "raises an error if default is inconsistent with type" do
-    expect do
-      option("foo", :type => :numeric, :default => "bar")
-    end.to raise_error(ArgumentError, "An option's default must match its type.")
+    expect(capture(:stderr) do
+      option("foo_bar", :type => :numeric, :default => "baz")
+    end.chomp).to eq('Expected numeric default value for \'--foo-bar\'; got "baz" (string)')
   end
 
   it "does not raises an error if default is an symbol and type string" do