coverage: fix loaded function name

Now we load flatten functions for spaces with `@flatten` source. Source
starting with @ assumes that it describes the file name, yet here it
isn't. Then luacov fails to find "flatten" file on coverage info build.

When we do not specify source explicitly, the whole string becames the
source. It will not confuse luacov anymore (it just ignores them), and
it's more helpful for debug.

1. https://www.lua.org/manual/5.2/manual.html#pdf-load

Closes #249

CHANGELOG.md

@@ -10,6 +10,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Changed
 * Yield on select/pairs storage tuple lookup (#312).
 
+### Fixed
+* Loaded functions misleading coverage (#249).
+
 ## [1.1.0] - 13-03-23
 
 ### Added

crud/common/utils.lua

@@ -217,7 +217,7 @@ function utils.flatten(object, space_format, bucket_id, skip_nullability_check)
         fieldmap = fieldmap,
         NULL = box.NULL,
     }
-    flatten_func = assert(load(code, '@flatten', 't', env))
+    flatten_func = assert(load(code, nil, 't', env))
 
     flatten_functions_cache[space_format] = flatten_func
     local data, err = flatten_func(object, bucket_id, skip_nullability_check)