Going by the spec, it's not a bug. I had to fix the missing quote in your HTML and the incorrect argument to click_button in your code to test it ( since fixed in the OP ), but mech does perform the prescribed action:

When a pointing device is used to click on the image, the form is submitted and the click coordinates passed to the server. The x value is measured in pixels from the left of the image, and the y value in pixels from the top of the image. The submitted data includes name.x=x-value and name.y=y-value where "name" is the value of the name attribute, and x-value and y-value are the x and y coordinate values, respectively.

Specifically, it sends some.x=1 and some.y=1 as it should. Sending some=thisvalue would be wrong.

The HTML5 spec clarifies many ambiguities in HTML4. It says the following the values of image buttons must be:

A coordinate, relative to a particular image's size, with the extra semantic that it must be the last value selected and initiates form submission

The table is very specific and comprehensive. Not only shouldn't "thisvalue" be sent, it isn't even a valid value for a <input type="image">.

Update: Added HTML5's view.

Update: Interestingly, IE is actually following the spec, and FF isn't.