Ignore a leading UTF-8 BOM in an SSE stream

The SSE spec strips one BOM at the start of the stream, but
SSEClient kept it as part of the first field name, so the
first event was silently dropped.

test/test.cc

@@ -22793,6 +22793,16 @@ TEST_F(SSEParsingTest, FieldNameOnlyWithCarriageReturn) {
   EXPECT_EQ(msgs[0].data, "");
 }
 
+TEST_F(SSEParsingTest, LeadingByteOrderMarkIgnored) {
+  // Only a BOM at the start of the stream is ignored
+  auto msgs = parse("\xEF\xBB\xBF"
+                    "data: first\n\n"
+                    "\xEF\xBB\xBF"
+                    "data: second\n\n");
+  ASSERT_EQ(msgs.size(), 1u);
+  EXPECT_EQ(msgs[0].data, "first");
+}
+
 TEST_F(SSEParsingTest, UnknownFieldsIgnored) {
   auto msgs = parse("unknown: value\ndata: hello\n\n");
   ASSERT_EQ(msgs.size(), 1u);