Ensure all brackes are decoded in JSON based Body Templates

Fixes OpenFeign#1129

When JSON is detected in a Body Template, all start and end tokens
that may have been pct-encoded are decoded, ensuring that the
expanded result is valid JSON.

core/src/main/java/feign/template/BodyTemplate.java

@@ -50,14 +50,9 @@ private BodyTemplate(String value, Charset charset) {
   public String expand(Map<String, ?> variables) {
     String expanded = super.expand(variables);
     if (this.json) {
-      /* decode only the first and last character */
-      StringBuilder sb = new StringBuilder();
-      sb.append(JSON_TOKEN_START);
-      sb.append(expanded,
-          expanded.indexOf(JSON_TOKEN_START_ENCODED) + JSON_TOKEN_START_ENCODED.length(),
-          expanded.lastIndexOf(JSON_TOKEN_END_ENCODED));
-      sb.append(JSON_TOKEN_END);
-      return sb.toString();
+      /* restore all start and end tokens */
+      expanded = expanded.replaceAll(JSON_TOKEN_START_ENCODED, JSON_TOKEN_START);
+      expanded = expanded.replaceAll(JSON_TOKEN_END_ENCODED, JSON_TOKEN_END);
     }
     return expanded;
   }

core/src/test/java/feign/template/BodyTemplateTest.java

@@ -0,0 +1,19 @@
+package feign.template;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Collections;
+import org.junit.Test;
+
+public class BodyTemplateTest {
+
+  @Test
+  public void bodyTemplatesSupportJsonOnlyWhenEncoded() {
+    String bodyTemplate = "%7B\"resize\": %7B\"method\": \"fit\",\"width\": {size},\"height\": {size}%7D%7D";
+    BodyTemplate template = BodyTemplate.create(bodyTemplate);
+    String expanded = template.expand(Collections.singletonMap("size", "100"));
+    assertThat(expanded)
+        .isEqualToIgnoringCase(
+            "{\"resize\": {\"method\": \"fit\",\"width\": 100,\"height\": 100}}");
+  }
+}