Fix UTF-8 encoding issue with binary attachment content in JSON serialization

- Add normalize_json_encodings! method to handle binary attachment content
- Automatically base64 encode binary strings before JSON serialization
- Fixes 'source sequence is illegal/malformed utf-8' error when sending
  attachments with raw binary content via JSON (non-multipart) requests
- Preserves backward compatibility with both symbol and string keys
- Resolves issue #528

Author: AaronDDM

lib/nylas/handler/http_client.rb

@@ -103,6 +103,7 @@ def build_request(
       is_multipart = !payload.nil? && (payload["multipart"] || payload[:multipart])
 
       if !payload.nil? && !is_multipart
+        normalize_json_encodings!(payload)
         payload = payload&.to_json
         resulting_headers["Content-type"] = "application/json"
       elsif is_multipart
@@ -257,6 +258,31 @@ def normalize_multipart_encodings!(payload)
       end
     end
 
+    # Normalize JSON encodings for attachment content to ensure binary data is base64 encoded.
+    # This handles cases where users pass raw binary content directly instead of file objects.
+    def normalize_json_encodings!(payload)
+      return unless payload.is_a?(Hash)
+
+      # Handle attachment content encoding for JSON serialization
+      attachments = payload[:attachments] || payload["attachments"]
+      return unless attachments
+
+      attachments.each do |attachment|
+        content = attachment[:content] || attachment["content"]
+        next unless content.is_a?(String)
+
+        # If content appears to be binary (non-UTF-8), base64 encode it
+        next unless content.encoding == Encoding::ASCII_8BIT || !content.valid_encoding?
+
+        encoded_content = Base64.strict_encode64(content)
+        if attachment.key?(:content)
+          attachment[:content] = encoded_content
+        else
+          attachment["content"] = encoded_content
+        end
+      end
+    end
+
     # Create a StringIO object that behaves more like a File for HTTParty compatibility
     def create_file_like_stringio(content)
       # Content is already normalized to ASCII-8BIT by normalize_multipart_encodings!