Merge pull request #210 from mwarzybok-sumoheavy/feature/SP-398

SP-398 Java: Add ability to create an invoice with a custom GUID

Author: bobbrodie

src/main/java/com/bitpay/sdk/Client.java

@@ -306,7 +306,9 @@ public Invoice createInvoice(Invoice invoice) throws InvoiceCreationException {
      */
     public Invoice createInvoice(Invoice invoice, String facade, Boolean signRequest) throws BitPayException, InvoiceCreationException {
         invoice.setToken(this.getAccessToken(facade));
-        invoice.setGuid(this.getGuid());
+        if (invoice.getGuid().equals("")) {
+            invoice.setGuid(this.getGuid());
+        }
         ObjectMapper mapper = new ObjectMapper();
         String json;