Who coined the phrase 'Ignorance is Bliss'?

This phrase is commonly thought to have originated in the 1742 poem titled, "Ode on a Distant Prospect of Eton College" by Thomas Gray.

Lines 91-100

To each his sufferings: all are men,
Condemned alike to groan;
The tender for another's pain,
The unfeeling for his own.
Yet ah! why should they know their fate?
Since sorrow never comes too late,
And happiness too swiftly flies.
Thought would destroy their paradise.
No more; where ignorance is bliss,
'Tis folly to be wise.