Fix popState to honor expression start char

Fixes #647

Author: mtdowling

smithy-utils/src/main/java/software/amazon/smithy/utils/CodeWriter.java

@@ -650,11 +650,11 @@ public CodeWriter popState() {
                 // back into a parent state.
                 popped.builder.setLength(0);
                 popped.builder.append(result);
-            } else {
+            } else if (!result.isEmpty()) {
                 // Sections can be added that are just placeholders. In those cases,
                 // do not write anything unless the section emitted a non-empty string.
-                // Note that this has already been formatted, so escape "$".
-                writeOptional(result.replace("$", "$$"));
+                // This has already been formatted, so writeWithNoFormatting is used.
+                writeWithNoFormatting(result);
             }
         }
 

smithy-utils/src/test/java/software/amazon/smithy/utils/CodeWriterTest.java

@@ -538,6 +538,21 @@ public void poppedSectionsEscapeDollars() {
         assertThat(result, equalTo("$Hello\n"));
     }
 
+    @Test
+    public void poppedSectionsEscapeCustomExpressionStarts() {
+        CodeWriter writer = CodeWriter.createDefault();
+        String result = writer
+                .setExpressionStart('#')
+                .pushState("foo")
+                .write("##Hello")
+                .write("$Hello")
+                .write("$$Hello")
+                .popState()
+                .toString();
+
+        assertThat(result, equalTo("#Hello\n$Hello\n$$Hello\n"));
+    }
+
     @Test
     public void canWriteInline() {
         String result = CodeWriter.createDefault()