fix for webmock request body expecting a string

when building the request signature, the body is preemptively converted to a string, which fulfills the expectation for webmock, despite it being a bit of a perf penalty if the request contains a multipart request body, as the body will be fully read to memory

Closes #319

Closes https://github.com/HoneyryderChuck/httpx/issues/65

integration_tests/webmock_test.rb

@@ -155,6 +155,12 @@ class WebmockTest < Minitest::Test
     assert_requested(:get, MOCK_URL_HTTP, query: hash_excluding("a" => %w[b c]))
   end
 
+  def test_verification_that_expected_request_with_hash_as_body
+    stub_request(:post, MOCK_URL_HTTP).with(body: { foo: "bar" })
+    http_request(:post, MOCK_URL_HTTP, form: { foo: "bar" })
+    assert_requested(:post, MOCK_URL_HTTP, body: { foo: "bar" })
+  end
+
   def test_verification_that_non_expected_request_didnt_occur
     expected_message = Regexp.new(
       "The request GET #{MOCK_URL_HTTP}/ was not expected to execute but it executed 1 time\n\n" \

lib/httpx/adapters/webmock.rb

@@ -20,7 +20,7 @@ module WebMock
           WebMock::RequestSignature.new(
             request.verb.downcase.to_sym,
             uri.to_s,
-            body: request.body,
+            body: request.body.to_s,
             headers: request.headers.to_h
           )
         end

lib/httpx/transcoder/multipart/encoder.rb

@@ -18,6 +18,12 @@ module HTTPX
         "multipart/form-data; boundary=#{@boundary}"
       end
 
+      def to_s
+        read
+      ensure
+        rewind
+      end
+
       def read(length = nil, outbuf = nil)
         data   = String(outbuf).clear.force_encoding(Encoding::BINARY) if outbuf
         data ||= "".b

sig/transcoder/multipart.rbs

@@ -28,6 +28,8 @@ module HTTPX
 
         def content_type: () -> String
 
+        def to_s: () -> String
+
         def read: (?int? length, ?string? buffer) -> String?
 
         def rewind: () -> void