chainhash: JSON Unmarshal hash from appropriate string.

[LindenWang01]

This commit(1d6e578) occurred a problem:
"error": "json: cannot unmarshal string into Go struct field OutPoint.TxIn.PreviousOutPoint.Hash of type chainhash.Hash"

adds a UnmarshalJSON() method to chainhash.Hash to fix it.

[coveralls]

Pull Request Test Coverage Report for Build 4258913166

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.006%) to 55.281%

---

Files with Coverage Reduction	New Missed Lines	%
connmgr/connmanager.go	3	86.27%

Totals
Change from base Build 4163994991:	0.006%
Covered Lines:	26641
Relevant Lines:	48192

---

💛 - Coveralls

[kcalvinalvin]

How about a simple test in chaincfg/chainhash/hash_test.go to catch these bugs in the future?

Maybe something like:

func TestHashJsonMarshal(t *testing.T) {
        tests := []struct {
                hash Hash
                err  error
        }{
                // Genesis hash.
                {
                        mainNetGenesisHash,
                        nil,
                },
        }

        unexpectedErrStr := "TestHashJsonMarshal #%d got an unexpected error: %v"
        unexpectedResultStr := "TestHashJsonMarshal #%d got: %v want: %v"
        for i, test := range tests {
                bytes, err := test.hash.MarshalJSON()
                if err != nil {
                        t.Errorf(unexpectedErrStr, i, err)
                        continue
                }

                got := new(Hash)
                err = json.Unmarshal(bytes, got)
                if err != nil {
                        t.Errorf(unexpectedErrStr, i, err)
                        continue
                }

                if !test.hash.IsEqual(got) {
                        t.Errorf(unexpectedResultStr, i, got, &test.hash)
                        continue
                }
        }
}

[Roasbeef]

Approved the CI run.

[jcvernaleo]

OK

[kcalvinalvin]

I see no reason for the holdup for this PR. Anything else to address? @jcvernaleo @Roasbeef

[jcvernaleo]

@kcalvinalvin I have no idea why I approved but didn't merge. I must have gotten distracted or something. That delay's on me, sorry about that. Merging now.

[chappjc]

MarshalJSON was in chaincfg/chainhash/v1.0.2. If possible, it would be good to tag a v1.0.3 with the matching UnmarshalJSON method added in this PR.