Inconsistency while treating fields as numbers or strings when reading or writing them

[lgyurci]

Reproduction:

1. Create a test file (test.yaml) with the following content:

a: abc
b: cba

2. Change the value of "a" to the string "23695230e640":

yq -i -y '.a="23695230e640"' test.yaml

3. Observe that the value is correctly changed in the file:

» cat test.yaml
a: 23695230e640
b: cba

4. Make another change to a field other than "a":

yq -i -y '.b="abc"' test.yaml

5. Observe the file again, and notice how the field "a" got treated as a number in this case:

» cat test.yaml
a: 1.7976931348623157e+308
b: abc

This breaks some processes. The value of "a" should not change if only the value of "b" is changed, and "a" should be consistently treated either as a number, or as a string.