Fix errors when default external encoding is not UTF-8

This fixes 6 test errors/failures when the default external encoding
was set to US-ASCII instead of UTF-8.

test/tilt_erbtemplate_test.rb

@@ -56,7 +56,7 @@ class MockOutputVariableScope
   end
 
   test "backtrace file and line reporting without locals" do
-    data = File.read(__FILE__).split("\n__END__\n").last
+    data = File.read(__FILE__, :encoding=>'UTF-8').split("\n__END__\n").last
     fail unless data[0] == ?<
     template = Tilt::ERBTemplate.new('test.erb', 11) { data }
     begin
@@ -72,7 +72,7 @@ class MockOutputVariableScope
   end
 
   test "backtrace file and line reporting with locals" do
-    data = File.read(__FILE__).split("\n__END__\n").last
+    data = File.read(__FILE__, :encoding=>'UTF-8').split("\n__END__\n").last
     fail unless data[0] == ?<
     template = Tilt::ERBTemplate.new('test.erb', 1) { data }
     begin
@@ -158,7 +158,7 @@ class Scope
   end
 
   test "backtrace file and line reporting without locals" do
-    data = File.read(__FILE__).split("\n__END__\n").last
+    data = File.read(__FILE__, encoding: 'UTF-8').split("\n__END__\n").last
     fail unless data[0] == ?<
     template = Tilt::ERBTemplate.new('test.erb', 11) { data }
     begin
@@ -174,7 +174,7 @@ class Scope
   end
 
   test "backtrace file and line reporting with locals" do
-    data = File.read(__FILE__).split("\n__END__\n").last
+    data = File.read(__FILE__, encoding: 'UTF-8').split("\n__END__\n").last
     fail unless data[0] == ?<
     template = Tilt::ERBTemplate.new('test.erb') { data }
     begin

test/tilt_template_test.rb

@@ -306,6 +306,7 @@ def default_encoding
     end
 
     test "uses the template from the generated source code" do
+      Encoding.default_external = 'UTF-8'
       tmpl = "ふが"
       code = tmpl.inspect.encode('Shift_JIS')
       inst = DynamicMockTemplate.new(:code => code) { '' }
@@ -315,6 +316,7 @@ def default_encoding
     end
 
     test "uses the magic comment from the generated source code" do
+      Encoding.default_external = 'UTF-8'
       tmpl = "ふが"
       code = ("# coding: Shift_JIS\n" + tmpl.inspect).encode('Shift_JIS')
       # Set it to an incorrect encoding