added test for to_s rep of error response (i.e. must contain the canonical message class def)

lib/httpx/response.rb

@@ -274,7 +274,8 @@ module HTTPX
       end
     else
       def to_s
-        "#{@error.message} (#{@error.class})\n#{@error.backtrace.join("\n")}"
+        "#{@error.message} (#{@error.class})\n" \
+        "#{@error.backtrace.join("\n") if @error.backtrace}"
       end
     end
 

test/error_response_test.rb

@@ -5,18 +5,18 @@ require_relative "test_helper"
 class ErrorResponseTest < Minitest::Test
   include HTTPX
 
-  def test_response_status
+  def test_error_response_status
     r1 = ErrorResponse.new(request_mock, RuntimeError.new("wow"), {})
     assert r1.status == "wow"
   end
 
-  def test_response_raise_for_status
+  def test_error_response_raise_for_status
     some_error = Class.new(RuntimeError)
     r1 = ErrorResponse.new(request_mock, some_error.new("wow"), {})
     assert_raises(some_error) { r1.raise_for_status }
   end
 
-  def test_respond_method_missing_errors
+  def test_error_response_respond_method_missing_errors
     r1 = ErrorResponse.new(request_mock, RuntimeError.new("wow"), {})
     ex1 = assert_raises(NoMethodError) { r1.read }
     assert ex1.message =~ /undefined response method/
@@ -24,6 +24,12 @@ class ErrorResponseTest < Minitest::Test
     assert ex2.message =~ /undefined method/
   end
 
+  def test_error_response_to_s
+    r = ErrorResponse.new(request_mock, RuntimeError.new("wow"), {})
+    str = r.to_s
+    assert str.match(/wow \(.*RuntimeError.*\)/), "expected \"wow (RuntimeError)\" in \"#{str}\""
+  end
+
   private
 
   def request_mock