Engine: Fix ERB expression compilation when code contains a heredoc (#1206)

When an ERB output tag contains a heredoc (e.g. `<%= method_call
<<~GRAPHQL, variables %>`), the closing parenthesis in the compiled Ruby
must appear on its own line after the heredoc terminator. Without this,
the compiled Ruby is syntactically invalid.

This is a sibling commit to the one introduced in
marcoroth/reactionview#78

Author: pinzonjulian

lib/herb/engine.rb

@@ -214,13 +214,17 @@ def add_expression(indicator, code)
 
     def add_expression_result(code)
       with_buffer {
-        @src << " << (" << code << comment_aware_newline(code) << ").to_s"
+        @src << " << (" << code
+        @src << "\n" if heredoc?(code)
+        @src << comment_aware_newline(code) << ").to_s"
       }
     end
 
     def add_expression_result_escaped(code)
       with_buffer {
-        @src << " << " << @escapefunc << "((" << code << comment_aware_newline(code) << "))"
+        @src << " << " << @escapefunc << "((" << code
+        @src << "\n" if heredoc?(code)
+        @src << comment_aware_newline(code) << "))"
       }
     end
 
@@ -248,6 +252,10 @@ def comment_aware_newline(code)
       code.include?("#") ? "\n" : ""
     end
 
+    def heredoc?(code)
+      code.match?(/<<[~-]?\s*['"`]?\w/)
+    end
+
     def add_postamble(postamble)
       terminate_expression
       @src << postamble

sig/herb/engine.rbs

@@ -164,5 +164,18 @@ class EngineTest < Minitest::Spec
 
       assert_compiled_snapshot(template, escape: false)
     end
+
+    test "heredoc with trailing arguments compiles to valid Ruby" do
+      template = <<~ERB
+        <%= method_call <<~GRAPHQL, variables
+          query {
+            field
+          }
+        GRAPHQL
+        %>
+      ERB
+
+      assert_compiled_snapshot(template)
+    end
   end
 end