Fix block_caller

* First wait the Thread start, then wait until it blocks or finishes.
* The bug would be to quit as soon as started is set, even if the Thread
  never blocked.
* Fixes #39.

lib/mspec/matchers/block_caller.rb

@@ -10,7 +10,11 @@ def matches?(block)
       blocking = false
     end
 
-    while !started and status = thread.status and status != "sleep"
+    Thread.pass while !started
+
+    # Wait until the Thread status is "sleep" (then it's blocking)
+    # or nil (the Thread finished execution, it did not block)
+    while status = thread.status and status != "sleep"
       Thread.pass
     end
     thread.kill