feat(restheadspec): match NULL when x-fieldfilter compares a numeric column to 0

Adds a SetNullAsZero handler option: on by default in restheadspec (for
x-fieldfilter only), off by default in resolvespec. When enabled, an eq
filter of 0 on a numeric column renders (col = 0 OR col IS NULL).

pkg/common/types.go

@@ -87,6 +87,10 @@ type FilterOption struct {
 	Operator      string      `json:"operator"`
 	Value         interface{} `json:"value"`
 	LogicOperator string      `json:"logic_operator"` // "AND" or "OR" - how this filter combines with previous filters
+
+	// NullAsZero makes an "eq" filter on a numeric column with a zero value also
+	// match NULLs (COALESCE(col, 0) = 0). Set for x-fieldfilter headers only.
+	NullAsZero bool `json:"-"`
 }
 
 type SortOption struct {