Avoid panic on pretty string ending in single quote

Fixes #262.

src/ser.rs

@@ -585,6 +585,10 @@ impl<'a> Serializer<'a> {
                     }
                 }
             }
+            if can_be_pretty && found_singles > 0 && value.ends_with('\'') {
+                // We cannot escape the ending quote so we must use """
+                can_be_pretty = false;
+            }
             if !can_be_pretty {
                 debug_assert!(ty != Type::OnelineTripple);
                 return Repr::Std(ty);

test-suite/tests/valid.rs

@@ -250,3 +250,7 @@ test!(table_array_nest_no_keys,
 test!(dotted_keys,
       include_str!("valid/dotted-keys.toml"),
       include_str!("valid/dotted-keys.json"));
+
+test!(quote_surrounded_value,
+      include_str!("valid/quote-surrounded-value.toml"),
+      include_str!("valid/quote-surrounded-value.json"));

test-suite/tests/valid/quote-surrounded-value.json

@@ -0,0 +1,10 @@
+{
+  "double": {
+    "type": "string",
+    "value": "\"double quotes here\""
+  },
+  "single": {
+    "type": "string",
+    "value": "'single quotes here'"
+  }
+}

test-suite/tests/valid/quote-surrounded-value.toml

@@ -0,0 +1,2 @@
+double = '"double quotes here"'
+single = "'single quotes here'"