PostPolicyV4 classes could be improved

[dmitry-fa]

Classes defined in PostPolictyV4.java are not as perfect as they can be.
Enhancements and bugs are listed below:

class PostPolicyV4

• the class description is stingy, need to put more words and point to usage example
• public methods miss Javadoc
• no verification for given URL string is a valid URL
• BUG: getFields() returns the map as is, so one can modify PostPolicyV4 objects after creation
  (suggested fix: make fields immutable)

class PostPolicyV4.PostFieldsV4

• BUG: public method getFieldsMap() returns map that allows to alter PostFieldsV4 instances without Builder (no control)
  (suggested fix: deprecate method getFieldsMap(), make the map immutable, define getters)

class PostPolicyV4.PostConditionsV4

• BUG getConditions() returns mutable Set
• BUG getConditions() exposes non public class PostPolicyV4.ConditionV4
  (suggested fix: make PostPolicyV4.ConditionV4 public, make conditions immutable)

[dmitry-fa]

@frankyn could you look at problems listed in the description and say what you think about them.

[frankyn]

Good catches @dmitry-fa, can you move forward if time permits?

[dmitry-fa]

@frankyn, sure I'll fix that issue. And then cover that functionality with tests (in a separate PR)

[dmitry-fa]

@frankyn I found another issue which I don't know how to address: when I specify the content-length the POST request fails with with "400 Bad Request"
I use ITStorageTest.testSignedPostPolicyV4 to play with post policy and I just updated
PostFieldsV4 fields = PostFieldsV4.newBuilder().setAcl("public-read").build();
to
PostFieldsV4 fields = PostFieldsV4.newBuilder().setAcl("public-read").setContentLength(11).build();

Should it be setContentLengthRange(from, to) instead?

[dmitry-fa]

https://cloud.google.com/storage/docs/authentication/signatures#policy-document states:
All valid HTML form fields except Content-Length can use exact matching.

[frankyn]

ContentLength range support is in conditional policy:

java-storage/google-cloud-storage/src/main/java/com/google/cloud/storage/PostPolicyV4.java

Lines 270 to 273 in 02dc3b5

	public Builder addContentLengthRangeCondition(int min, int max) {
	conditions.add(new ConditionV4(ConditionV4Type.CONTENT_LENGTH_RANGE, "" + min, "" + max));
	return this;
	}

ContentLength in the Policy is wrong and should be deprecated with a note.