Update dirty flag on modifying section values

[naveenjr7]

Today when we make any changes to any section in Gppmodel object, its not getting reflected while we encoding back.
And root cause for this issue is, dirty flag is not getting updated properly on setFieldValue method.

@chuff could you please review this.

Issue can be reproduced with below steps

GppModel gppModel = new GppModel("DBABBg~BVoIgACY.QA");
// Ideally, this should cause validation failure
// Reference: https://github.com/IABTechLab/iabgpp-java/blob/fcd606121f5795080620099a9dc51dac0482f46f/iabgpp-encoder/src/main/java/com/iab/gpp/encoder/segment/UsCaV1CoreSegment.java#L110-L115
gppModel.setFieldValue(UsCaV1.NAME, UsCaV1Field.SHARING_OPT_OUT_NOTICE, 0);
gppModel.setFieldValue(UsCaV1.NAME, UsCaV1Field.SHARING_OPT_OUT, 1);
String encodedString = gppModel.encode();

[naveenjr7]

thanks for the approval. @iabmayank could you please help merging this fix