Fix entries API POST response format and empty array handling

Fixes two bugs in the entries API that caused test failures:

1. POST endpoints now return JSON arrays consistently
   - Created format_post_response() middleware for POST requests
   - Replaces format_entries() which is designed for GET with content negotiation
   - Previously, POST requests without Accept header defaulted to text/plain handler
   - This caused responses to fail or return empty objects instead of JSON arrays
   - Now matches behavior of treatments and devicestatus APIs

2. Fixed callback never being called for empty array posts
   - Added empty array check in lib/server/entries.js create() function
   - Previously, empty array caused forEach loop to never execute
   - Completion callback was inside forEach, so never triggered for empty input
   - This caused 15 second timeouts on POST requests with empty arrays

All 26 tests in api.shape-handling.test.js now pass.

Files changed:
- lib/api/entries/index.js: Added format_post_response, updated POST routes
- lib/server/entries.js: Added empty array handling in create()

lib/api/entries/index.js

@@ -252,6 +252,22 @@ function configure (app, wares, ctx, env) {
     });
   }
 
+  /**
+   * @method format_post_response
+   * Simple middleware to format POST response as JSON array
+   * Unlike format_entries, this doesn't support content negotiation
+   * and always returns JSON, which is appropriate for POST responses
+   */
+  function format_post_response (req, res) {
+    // If there's been some error, report that
+    if (res.entries_err) {
+      return res.sendJSONStatus(res, consts.HTTP_INTERNAL_ERROR, 'Mongo Error', res.entries_err);
+    }
+    
+    // Always return JSON array for POST requests
+    res.json(res.entries || []);
+  }
+
   /**
    * @method insert_entries
    * middleware to process "uploads" of sgv data
@@ -767,7 +783,7 @@ function configure (app, wares, ctx, env) {
     // setting this flag tells insert_entries to not actually store the results
     req.persist_entries = false;
     next();
-  }, insert_entries, wares.obscure_device, format_entries);
+  }, insert_entries, wares.obscure_device, format_post_response);
 
   // Protect endpoints with authenticated api.
   if (app.enabled('api')) {
@@ -782,7 +798,7 @@ function configure (app, wares, ctx, env) {
       // setting this flag tells insert_entries to store the results
       req.persist_entries = true;
       next();
-    }, insert_entries, wares.obscure_device, format_entries);
+    }, insert_entries, wares.obscure_device, format_post_response);
 
     /**
      * @module delete#/entries/:spec

lib/server/entries.js

@@ -95,6 +95,11 @@ function storage (env, ctx) {
       , numDocs = docs.length
       , totalCreated = 0;
 
+    // Handle empty array case - call callback immediately
+    if (numDocs === 0) {
+      return fn(null, docs);
+    }
+
     docs.forEach(function(doc) {
 
       // Normalize dates to be in UTC, store offset in utcOffset