Fix will_fork? handling for before_fork hooks

The intended behavior of before_fork, according to a comment:

lib/resque/worker.rb
258:      # Only run before_fork hooks if we're actually going to fork

If this is the case, then this behavior was actually broken on e.g.
JRuby, as there was no guard on the invocation of the hook.

This adds that guard, and cleans up the tests to check will_fork? before
making assertions.

Author: tarcieri

lib/resque/worker.rb

@@ -257,7 +257,7 @@ def fork(job)
 
       # Only run before_fork hooks if we're actually going to fork
       # (after checking @cant_fork)
-      run_hook :before_fork, job
+      run_hook :before_fork, job if will_fork?
 
       begin
         # IronRuby doesn't support `Kernel.fork` yet

test/resque_hook_test.rb

@@ -44,7 +44,7 @@ def self.perform
 
     assert_equal(0, counter)
     @worker.work(0)
-    assert_equal(2, counter)
+    assert_equal(@worker.will_fork? ? 2 : 0, counter)
   end
 
   it 'calls after_fork after each job if forking' do
@@ -102,7 +102,12 @@ def self.perform
 
     assert(!first && !second)
     @worker.work(0)
-    assert(first && second)
+
+    if @worker.will_fork?
+      assert(first && second)
+    else
+      assert(!first && !second)
+    end
   end
 
   it 'registers multiple after_forks' do

test/worker_test.rb

@@ -447,7 +447,7 @@ def self.perform
     assert !$BEFORE_FORK_CALLED
     Resque::Job.create(:jobs, SomeJob, 20, '/tmp')
     workerA.work(0)
-    assert $BEFORE_FORK_CALLED
+    assert $BEFORE_FORK_CALLED == workerA.will_fork?
   end
 
   it "Will not call a before_fork hook when the worker can't fork" do