Fix schemas for bigdecimal/rust_decimal (#248)

Both of these crates serialise decimals as strings, not as plain numbers

schemars/tests/decimal.rs

@@ -0,0 +1,12 @@
+mod util;
+use util::*;
+
+#[test]
+fn rust_decimal() -> TestResult {
+    test_default_generated_schema::<rust_decimal::Decimal>("rust_decimal")
+}
+
+#[test]
+fn bigdecimal() -> TestResult {
+    test_default_generated_schema::<bigdecimal::BigDecimal>("bigdecimal")
+}

schemars/tests/expected/bigdecimal.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "Decimal",
+  "type": "string",
+  "pattern": "^-?[0-9]+(\\.[0-9]+)?$"
+}

schemars/tests/expected/rust_decimal.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "Decimal",
+  "type": "string",
+  "pattern": "^-?[0-9]+(\\.[0-9]+)?$"
+}