Fix issue #43: don't alter array passed to method.

Reported by github.com/joelmats. The array passed to
Geocoder::Calculations.geographic_center was being modified in-place.

lib/geocoder/calculations.rb

@@ -120,7 +120,7 @@ module Geocoder
     def geographic_center(points)
 
       # convert objects to [lat,lon] arrays and remove nils
-      points.map!{ |p| p.is_a?(Array) ? p : p.to_coordinates }.compact
+      points = points.map{ |p| p.is_a?(Array) ? p : p.to_coordinates }.compact
 
       # convert degrees to radians
       points.map!{ |p| to_radians(p) }

test/geocoder_test.rb

@@ -51,6 +51,14 @@ class GeocoderTest < Test::Unit::TestCase
     assert Geocoder.address(40.750354, -73.993371).is_a?(String)
   end
 
+  def test_geographic_center_doesnt_overwrite_argument_value
+    # this tests for the presence of a bug that was introduced in version 0.9.11
+    orig_points = [[52,8], [46,9], [42,5]]
+    points = orig_points.clone
+    Geocoder::Calculations.geographic_center(points)
+    assert_equal orig_points, points
+  end
+
 
   # --- general ---